Bold italic capital Greek letters in maths

I'm using R and Bookdown. In the html output I'd like to have an inline boldface italic capital Greek letter. I've tried a variety of inputs, for example

...some text $\bm{\varGamma}$ some text...

...some text $\boldsymbol{\varGamma}$ some text...

...some text $\mathbf{\varGamma}$ some text...

This unanswered question asks about boldface Greek, but the suggestion in the comments of using \mathbf works only with \Gamma. I haven't found any questions about italic capital Greek letters.

Edit: the second input line above works, as explained in the accepted answer below. The real problem came from \varGamma which seems to cause trouble with Bookdown's epub output.

boldsymbol also works using html_document, but be aware of difference between bm and boldsymbols:

---
output: html_document
---

...some text **bold**: $\boldsymbol{\varGamma}$ some text... not bold: ${\varGamma}$.